ATLANTA — A three-state manhunt for the shooter in a fatal ambush attack on a state highway patrol barracks in Dunmore, Penn., on Friday night underscores a troubling rise in the number of surprise attacks on police officers throughout the US in the past few years.

One trooper was killed and another injured after someone opened fire just outside the gates of the barracks during a shift change late Friday evening. State Police Commissioner Frank Noonan said the attack appeared to be solely directed at state police.

Police say a massive manhunt has netted one “person of interest” so far, but that person has not been charged with any crime. “We can’t say that the situation is completely in hand,” Mr. Noonan told reporters Saturday morning.

The National Law Enforcement Officers Fund, which tallies the numbers of officers killed on duty, began sounding an alarm four years ago about a rise in the number of ambush attacks on police officers.

A series of high-profile “movie-style” ambushes targeted members of the justice system in 2013, when a judge, prosecutor, and prison warden were gunned down in planned attacks. Earlier this week, Merrillville, Ind., officer Nickolaus Schultz was shot and killed in an apparent ambush attack by a man who subsequently killed himself.

A USA Today review in 2011 showed that nearly 40 percent of officer deaths were from ambush attacks, up from 31 percent two years earlier.

Ambush attacks are "generally accomplished from cover or hiding; however, they can occur without cover or hiding," the FBI says.

The Pennsylvania barracks shooting took place in a small town near Scranton. Police from across the region, including a police helicopter and SWAT teams, descended on the area to search for the shooter. Police said they had several leads. One of the troopers had just arrived at the barracks and the other was leaving when they were attacked. Authorities have not yet released the names of the troopers.

While significant, the number of ambush shootings of police today is down significantly from the 1970s, when attacks spiked in an era where there was generally less respect for police officers. “When respect goes up, the attacks go down,” former FBI chief Tom Fuentes told CNN last year.